Dear Tom,
Thank you so much for your advice. I will take a look at the SnPM.
Anyway, the SPM gave me the following:
Degree of freedom = [1.0, 4.0]
FWHM = 4.3 4.7 5.3 mm mm mm; 2.1 2.4 2.6 (voxels);
Volume: 1156096 = 144512 voxels = 8360.6 resels
Voxel size: 2.0 2.0 2.0 mm mm mm; (resel = 13.25 voxels)
Best,
Ji Hyun
From: [log in to unmask] [mailto:[log in to unmask]] On Behalf Of Thomas Nichols
Sent: Friday, May 21, 2010 4:16 AM
To: Ji Hyun Ko
Cc: [log in to unmask]
Subject: Re: [SPM] partial correlation for ligand PET study in SPM5
Dear Ji,
You are right to be skeptical of amazing results from just 7 subjects. Can you also report your estimated smoothness (FWHM in voxels and mm, and Resel count)? That will help to see if you have a chance of getting reasonable accuracy out of the RFT corrected P-values.
With such a tiny amount of data, you also might want to try a nonparametric approach, with variance smoothing in particular. See SnPM toolbox under SPM extensions for more details.
-Tom
On Thu, May 20, 2010 at 9:14 PM, Ji Hyun Ko <[log in to unmask]> wrote:
Dear SPM users,
I have two questions regarding how to do the partial correlation for a
ligand PET study.
I have 7 patients and each of them had one FLB-PET scan. I produced binding
potential maps (3D image) using some in-house software.
I have two covariates. One of the covariates is the behavioural score and
the other is age which should be controlled for (nuisance covariate).
So, I have design matrix of X = [behaviour, age, constant].
Of course, Those are column vectors.
And, I used C = [1 0 0].
My first question is...is this the right way to do partial correlation?
My second question is about extent threshold.
I put 0.001 for uncorrected height threshold, and put 10 for extent
threshold.
And, I think the SPM gives me some unrealistic values.
All of the p_corrected in cluster-level was less than 0.001.
And, in the left bottom of the result window, it gives:
Height threshold: T=7.17, p=0.001 (1.000) {p<0.001 (unc.)}
Extent threshold: k = 10 voxels, p=0.000 (0.000)
Expected voxels per cluster, <k>=0.156
Expected number of clusters, <c>=0.00
Expected false discovery rate, <=0.15
I think there is something wrong.
How can I have such "good" p_corrected values with only 7 scans?
Please help...
Best,
Ji Hyun
____________________________________
Ji Hyun Ko, PhD
Post-doctoral Fellow,
Centre for Addiction and Mental Health &
Toronto Western Research Institute,
University of Toronto
Physical Address:
455 Spadina Ave., Suite 402
Toronto, Ontario, M5S 2G8
CANADA
Mailing Address:
PET Centre, CAMH
250 College street
Toronto, Ontario, M5T 1R8
CANADA
Phone: +1-416-535-8501 ext. 7396
Fax: +1-416-979-3855
--
____________________________________________
Thomas Nichols, PhD
Principal Research Fellow, Head of Neuroimaging Statistics
Department of Statistics & Warwick Manufacturing Group
University of Warwick
Coventry CV4 7AL
United Kingdom
Email: [log in to unmask]
Phone, Stats: +44 24761 51086, WMG: +44 24761 50752
Fax: +44 24 7652 4532